The conversion of naphthas, which, typically, begin to boil within the gasoline boiling range, about 285.degree. F. (140.degree. C.), and finish boiling in the distillate range, e.g. 400.degree. F. (204.degree. C.), to materials which begin and end boiling within the gasoline boiling range, C.sub.5 to 330.degree. F., is important to refiners.
Aromatic heavy naphtha fractions such as fluid catalytically cracked (FCC) 300.degree.-425.degree. F. products are high in octane, but because they begin to boil above the gasoline end boiling range, and may contain a large proportion of sulfur impurities, they require further processing to become commercially valuable as gasoline. However, it is difficult to eliminate, or at least reduce, the properties which make them undesirable as gasoline, i.e. the high sulfur content and high boiling point, without compromising the high octane properties that make them desirable as gasoline.
Recently, it has been reported that lowering gasoline endpoint to achieve a product endpoint where, in a standard ASTM distillation, 90 volume percent of the gasoline distills below 300.degree. F. (T.sub.90) will reduce pollution. Meeting this T.sub.90 permits only 10% of the hydrocarbons in gasoline to boil above 300.degree. F. A significant boiling range conversion of heavy naphthas will be required to meet this goal.
U.S. Pat. No. 3,923,641 to Morrison discloses hydrocracking a C.sub.7 + naphtha over zeolite beta at moderate temperatures and pressures to achieve a high yield of iso-C.sub.4. However, the disclosure is silent on the yield and quality of higher hydrocarbons.
U.S. Pat. No. 4,812,223 discloses hydrocracking a C.sub.5 + naphtha over mildly steamed noble metal containing zeolite beta.
It is known that hydrocracking naphthas results in production of normal paraffins within the gasoline boiling range, particularly n-C.sub.5. It would be advantageous to suppress these reactions since n-paraffins have low octane numbers as compared to other gasoline boiling range components such as iso-paraffins.